Just transported my grandpa's 1966 F100 this spring break to my house so that I'll be able to work on it more than once or twice a year. Luckily a family friend gave me some pointers to get it running, but I'm having to charge the battery every other time I crank it. I have no idea how old the battery is, but doesn't look to be really old. Should I replace the battery or the alternator though? Both? This is the first auto work I've ever done aside from oil changes. Any ideas?

 

First welcome to FTE.
You can do a little testing if you have a multi meter.
Charge the battery fully get it running and up to temp.
Now hook the meter across the battery connections what reading do you get?
Now turn on the head lights & heater fan full blast, bring the motor RPM up to about 2500 RPM what reading do you get now?
I would say at any point if the readings, more so with everything turned on, the reading is below 13.5 volts the ALT is not charging.

Now there is a full field test you can do to see if the ALT is bad or the regulator but if you want to replace any of it I would do both at the same time because if 1 is bad it could take out the other 1.
If you want to do the full field test let us know and we can tell you how.

Now if the ALT is putting out 13.5 to 14.5 it is good and my guess the battery is NG. You can also fully charge the battery and take it to the auto parts store to see if they can test it but is has to be fully charged.
BTW what motor is in the truck?
Dave ----

 

Change the battery. If you don't know how old it is, it's too old. Testing is a good option, though my money says new battery, especially if it's been sitting for a long time.
Pull the alternator and have it tested, too. Then put it back with a new belt.
Anyone who tells you to change both alt and batt at once is a person spending other people's money, and is no mechanic.

Cosmo

 

Yeah, the battery probably should be changed, and Autozone and similar will check it for you for free, But you can check it yourself if you have a multimeter or a voltmeter. Here's how I would do it, although others may differ.


Without charging it first - If the disconnected battery tests less than 9.5 Volts, then it's got a dead cell - replace the battery.
You should check the battery water i(if it's not sealed) berfore charging to make sure it isn't dry.
After attempting to charge the battery,it reads 10.5 Volts or less, then it's got a dead cell.
If the battery around 12.7 Volts or so after charging, then it's starting to look good, but could be failing under load. Crank the engine with the meter across the terminals. It should drop to no less than around 9 volts. and then cllimb a little while cranking.
You can also let it set about 24 hours or at least overnight after charging and thencheck the voltage again. If it has dropped more then a maybe 2/10's of a volt in that short of a time, it should be replaced.
If it seems to be working well at this point, then Dave's advice above on checking the Alternator should get you going.

 

If your daily driver is a top post I'd probably swap it in, do what Dave said and see what happens. Like AZ66 said, the autoparts stores will test a battery for free, I think most will do alternators too.

 

Something to think about, in your kit a good basic battery charger should be among your first tool purchases. They will pay for themselves quickly. Keep the battery charged up 100% and they will last a lot longer, the alternator cannot do this, unless you drive to Montana or something like that.

 

I'll chime in here. The OP states he has to constantly charge his battery. If you can acquire a quality multimeter that can handle more than 10A unfused , you need to disconnect the positve battery lead and connect the multimeter in series between the cable and the positive post. With everything off and the key in your hand the meter should show 0 in the amp position. If you are getting half an amp or higher there is a draw on your battery which is draining it. If there is a draw then start disconnecting major components one at a time until the meter shows zero.

 

Thanks for all the help. Trying to take the battery to Autozone this weekend and get it tested. Will more than likely just save up to get both new at the same time.

 

Wasn't sure if I needed to just bring the alternator to get tested or needed it the whole truck for them to test. Thanks for the advice

 

I don't know what setup they have for checking the ALT?
When you take the battery in for testing, they will most likely need to charge it first before testing, you can ask about the ALT.
Again if you have a meter and with the motor running you can check the ALT out put. If below 13 volts the ALT and/or REG are bad.
As a guess the battery is bad and I would replace it and test from there.

My project was bought with a small tractor battery in it about 2 years old, way to small to stay in the truck when I get it on the road.
When it came time to start it after a cab off frame rebuild I charged the battery, all new cables and it fired right up.
I used my meter to make sure it was charging, I had to make a jumper harness for the ALT plug, found I had 2 wires mixed up and swapped them and it now shows charging on the meter.
I only start it to move the truck around as I am still working on it and the battery has not giving me any issues even as small as it is.

BTW if you charge the battery to check with a meter let it sit for 30 min.to an hour before using the meter. If you don't you can get a faults reading as it will show a surface charge of 13 volts or more.
Letting it sit the surface charge has time to dissipate, the battery should read 12.5 12.6 volts after sitting.
Dave ----

 

I watched a mechanic test the battery in the Ranger with a really nice Snap-On battery and charging system diagnostic tester. Wireless. I think he said it was about $900. Anyway it still basically just measured the voltages at certain points, e.g. battery at rest, during engine start, at idle in park, and then with accessories and headlights on at a fast idle, same as everyone did a hundred years ago.

Can do these same tests pretty much with any voltmeter maybe not so conveniently. Or get a printout. Turn the headlights on for about 5 minutes with engine off will remove a battery surface charge. Wait at least 10 minutes for the battery voltage to "bounce back" and then measure.

 

I bet if it was a young kid and you gave him a old meter he would not be able to run them same test or even know why he was running them.
Wonder who looks at the print out? The customer would not know what it was and cant see them getting it and why would the service manger what it?

I guess now days it is hard to get to stuff under the hood, would you even need to open it to check things?
Would think you could plug into the OBD port to get the information you are looking for no?

I have a Fox Valley meter I got back in the late 70's that will do every thing even amps if I can find the test wire that plugs into it.
It is still my go to meter when checking out a charging system other than amps because of the missing test wire.
Dave ----

 

Actually it was a graybeard running it. I think it was the 750 model. That model retail at $990 and are wireless and looks like it. Also checks for AC ripple from diode pack, that's handy.

Modern vehicle systems really munch on the battery after shutdown. So everything has to be in good running order, and then it barely keeps up, near as I can tell.

 

They can actually hook it up to the alt with it in the truck and test with the motor running. No need to remove it.

 

O'Reilly said they both past their test. I charged the battery overnight so I wouldn't have to wait for them to charge it before testing it. Hooked everything back up and can't get the motor to even try and turn over. No response at all when I turn the key. Checked all the connections with my shop manual and it all matched up. Last time I tried to start it, I got it to backfire before it did this same thing. I assumed it was the battery being dead. Any ideas?